Research conclusions: It is concluded that for the Sanjiadi seismic landslide in Longnan city，the slowly inclined structure of the loess mudstone interbedding is the internal factors of producing the landslide，its location in the joint zone of two structure is the internal power of producing the landslide，the abnormal development of debris flow is the main external factor of producing the landslide，and the coupling of seismic wave propagation direction with slope body stress field direction，the coupling of the seismic wave with the loess micro － structure and the coupling of seismic wave with groundwater anomalies are the factors of inducing the seismic loess landside． The proposal is made for control of the landslide according to formation mechanism． It has a certain guiding significance to the understanding of the formation mechanism of earthquake，the effective reduction of the secondary disasters induced by earthquake and the promotion of the post － disaster reconstruction work．